Effective treatment of thoracic trauma demands an in-depth knowledge of both anatomy and radiological imaging of the chest. Since relatively few major injuries are confined to a single body cavity, this knowledge must extend to the upper abdomen and root of the neck. Penetrating injuries in particular require detailed knowledge of the relationships between surface landmarks and intrathoracic and abdominal viscera. These change significantly during different phases of respiration.

The form of the thorax is that of a truncated cone, flattened in front and behind but rounded at the sides. The skeletal framework of the chest is formed anteriorly by the sternum and the costal cartilages, posteriorly by the bodies of the 12 thoracic vertebrae and the corresponding intervertebral discs, and by the ribs from their heads to their angles. On each side the chest is enclosed by the shafts of the ribs, from their angles to their cartilages. The sternum is made of three parts: the manubrium, body, and xiphoid process. The manubrium and body are not in the same plane, and thus form the sternal angle at their junction. This provides an important landmark, at which the second costal cartilate articulates with the sternum. The costal cartilages of the first to seventh ribs articulate with the sternum, whereas the costal cartilages of the eighth to tenth ribs usually attach to the cartilage of the rib above. The ventral ends of the cartilages of ribs 11 and 12 have no direct skeletal attachment. All ribs articulate dorsally with the vertebral column in such a way that their ventral end, together with the sternum can be elevated, as occurs during the inspiratory phase of respiration. The articulations of the costal cartilages with the sternum, apart from that of the first rib, are true synovial joints which allow freedom of movement.

The anterior wall of the chest is much shorter than is that of the posterior wall. During expiration the upper margin of the sternum is opposite the disc between the second and third thoracic vertebrae, and the xiphisternal joint is opposite the ninth or tenth thoracic vertebra. This relationship changes during inspiration so that the xiphisternal joint is elevated towards the eighth thoracic vertebra, and the suprasternal notch is raised to the level of the second thoracic vertebra. The ribs are, in general, directed downwards and forwards; during inspiration they become elevated and more horizontally placed by a hinge-like movement with its axis through the head and tubercle of the rib. Elevation is accompanied by lateral displacement which is inconspicuous in the uppermost ribs, but more pronounced in the fifth to ninth ribs (Fig. 1) 1908. Movement of the upper ribs increases the anteroposterior diameter of the chest, while movement of the lower ribs results mainly in an increase in transverse diameter. The short eleventh and twelfth ribs move mainly backwards in inspiration and forwards in expiration. Lateral movement of the lower ribs causes a widening of the infra-sternal angle, and permanent elevation of the ribs in the elderly produces the characteristic barrel-shaped chest. The thorax in a female is relatively shorter and more rounded than in the male, and the upper ribs are more mobile. These features provide greater movement of the upper chest, which is especially evident in the later stages of pregnancy.

The bodies of the thoracic vertebra project forward and greatly diminish the anteroposterior diameter of the chest in the median plane. The backward sweep of the posterior parts of the ribs produces a hollow on each side of the vertebral column which accommodates the greater part of the corresponding lung. Movements of the vertebral column contribute to changes in the volume of the thorax. The thoracic spine undergoes extension during inspiration. This causes still greater movement of the anterior ends of the upper ribs than would be produced by intrinsic movement relative to the vertebral column.

The inlet of the thorax is a narrow opening bounded by the body of the first thoracic vertebra, the first pair of ribs, and the upper border of the manubrium sterni. The plane of the inlet slopes very obliquely downwards and forwards, so that the anterior part of the apex of the lung is above the level of the anterior boundary of the inlet, though its posterior part only attains the level of the neck of the first rib. Patients with wounds at the root of the neck are therefore susceptible to pneumothorax. The structures that enter or leave the thorax through this inlet are the trachea, the oesophagus, the vagus and phrenic nerves on each side, the left recurrent laryngeal nerve, the thoracic duct, and the great arteries and veins carrying blood to and from the head, neck, and upper limbs. At the inlet, these are the right innominate artery and vein, the left subclavian artery, left common carotid artery, and left innominate vein (see Fig. 7 1914). The outlet of the thorax is much larger than the inlet. It is bounded by the xiphisternal joint, the lower six costal cartilages, the twelfth rib and the twelfth thoracic vertebra. The boundary of the outlet is curved, for it descends from the xiphisternum to the tip of the eleventh costal cartilage and then ascends towards the twelfth thoracic vertebra. The inner aspect of this lower margin gives attachment to the diaphragm, which forms a convex floor for the thorax and a concave roof for the abdomen. The upward bulging of the diaphragm greatly diminishes the vertical diameter of the thorax, so that the lower part of the chest greatly overlaps the upper part of the abdominal cavity, especially at the sides and behind (Fig. 2) 1909. There are three large openings in the diaphragm through which the aorta, oesophagus, and vena cava pass between the two cavities. The aortic opening transmits the aorta together with the thoracic duct and frequently the vena azygos as it ascends from the abdomen. The oesophageal opening transmits the oesophagus together with the vagus nerves. The vena caval opening transmits only this structure.

The deep surface of the scapula fits against the posterolateral aspect of the thorax from the second to seventh ribs. It is held in apposition by the muscles attached to it, the only bony articulation to the chest being between the acromion process and the lateral end of the clavicle. The clavicle acts as a strut to hold the lateral angle of the scapula away from the thorax. The clavicle articulates at its medial end with the superolateral aspect of the manubrium.

Deep to the skin and superficial fascia, which contains the mammary glands, the anterior chest wall is covered by three groups of muscles. These are the muscles of the upper extremity, those of the anterolateral abdominal wall, and the intrinsic muscles of the thorax (Fig. 3) 1910. The muscles of the upper extremity include the pectoralis major, pectoralis minor, serratus anterior, and subclavius. The pectoralis major and minor muscles are supplied by the medial and lateral anterior thoracic nerves, which are branches of the medial and lateral cords of the brachial plexus (Fig. 4) 1911. The serratus anterior muscle is supplied by the long thoracic nerve, which is a branch of the brachial plexus that courses inferiorly on the external surface of the muscle. This nerve may be damaged by penetrating wounds or chest drain insertion, causing winging of the scapula. The muscles of the anterolateral abdominal wall which originate on the chest are the external oblique muscle and the rectus abdominus muscle; both of these muscles are supplied by branches of the lower six thoracic nerves. When two or more of these branches are damaged at thoracotomy or during extensive thoracic injury, denervation of the muscles leads to ipsilateral bulging of the abdominal wall. The intrinsic muscles of the thorax include the external and internal intercostal muscles and the transversus thoracis muscle. The external intercostal muscles arise from the lower border of the rib above and insert on to the upper border to the rib below. Their fibres are directed downward and medially, extending from the tubercles of the ribs to the beginnings of the costal membranes. The internal intercostal muscles arise from the inner lip and floor of the costal groove of the rib above and from the related costal cartilage. They insert on to the upper border of the rib below. These muscles extend from the sternum to the angle of the ribs and then continue to the vertebral column as the posterior intercostal membranes. The fibres are directed downwards and laterally. Both external and internal intercostal muscles are supplied by the related intercostal nerves. The transversus thoracis muscle lines the inner surface of the anterior thorax wall as a thin sheet of muscular and tendinous fibres. The muscle arises from the posterior surfaces of the xiphoid process, the lower third of the body of the sternum, and the sternal ends of the related costal cartilages. It inserts by muscular strips into the inner surface of the second or third to the sixth costal cartilages.

Beneath the skin and superficial fascia of the dorsal aspect of the chest, a superficial group of muscles connect the upper limb to the vertebral column (Fig. 3) 1910. These are the trapezius, latissimus dorsi, rhomboids, and levator scapulae. The trapezius muscle arises from the posterior aspect of the skull, the posterior margin of the ligamentum nuchae which spans the spinous processes of the cervical vertebrae, and from all the thoracic vertebrae and their related supraspinous ligaments. The upper group of fibres inserts on to the lateral third of the clavicle; the middle group of fibres inserts on to the medial margin of the acromion process and upper margin of the posterior border of the spine of the scapula. The lower fibres converge into an aponeurosis, which slides over the triangular area of the medial end of the spine of the scapula and is attached at the apex of this triangle. When the muscle contracts the scapula is pulled medially, rotates, and carries the shoulder upwards. When the shoulder is fixed the upper fibres tilt the head, so that the face turns upwards towards the opposite side. The latissimus dorsi muscle is expansive, with a broad origin from the outer lip of the iliac crest and from an extensive aponeurosis which is attached to the spinous processes of the lower six thoracic vertebrae, the lumbar and sacral vertebrae, and their related supraspinous ligaments. The muscle is inserted into the humerus and helps produce extension, adduction, and medial rotation at the shoulder joint. It also helps to depress the raised arm against resistance. The rhomboid muscles originate from the spinous processes and supraspinous ligaments of the cervical and thoracic vertebrae, and insert into the vertebral border of the scapula. These muscles draw the scapula towards the vertebral column and also slightly upwards. The lower fibres of the major rhomboid muscle rotate the scapula and depress the shoulder. The levator scapulae muscle originates from the first four cervical vertebrae, inserts into the vertebral border of the scapula, and as its name suggests, elevates the scapula, drawing it medially and rotating it, so that the tip of the shoulder is depressed.

Deep to the muscles that connect the upper limb to the vertebral column lie the serratus posterosuperior and inferior muscles. The superior muscle helps increase the size of the thoracic cavity by elevating the ribs. The inferior muscle tends to pull the last four ribs downward and outward. Just deep to the serratus posterosuperior muscle lie the thoracic portions of the splenius, cervicus, and capitus muscles. The groove lateral to the spinous processes of the thoracic vertebrae is filled by the sacrospinalis muscle. This is covered by a strong layer of lumbodorsal fascia.

Surface landmarks on the chest wall are important when siting surgical incisions or placing intercostal drains. These landmarks may be obscured to a greater or lesser extent by body fat. The upper border of the manubrium, the sternal angle, and the xiphisternal joint can be felt and often seen in the majority of individuals. The second costal cartilage articulates with the sternum at the sternal angle. This arrangement facilitates identification of the second intercostal space. The seventh costal cartilage is usually the lowest to articulate with the sternum, although on occasions the eighth also reaches it. The tenth costal cartilage forms the lowest part of the costal margin, whilst the tips of the eighth, ninth, and tenth cartilages articulate with the anterior part of the costal margin, and can usually be felt easily. The eleventh and twelfth ribs are palpable posteriorly in slender subjects, but do not articulate with the costal margin.

In clinical practice the ribs are usually counted beginning with the second costal cartilage at the sternal angle. If the sternal angle cannot be felt, the first intercostal space is identified as the depression below the clavicle: the second rib lies immediately below it. The lower margin of the pectoralis major muscle overlaps the fifth rib and cartilage at about the same level as the xiphisternal joint. This site corresponds proximally to the level of the ninth or tenth thoracic vertebra. The upper end of the linea semulunaris of the abdomen originates at the tip of the ninth costal cartilage. This is usually about half way up the costal margin and at the level of the first lumbar vertebral. With the patient positioned for a lateral thoracotomy, the arm flexed and raised above the head, the tip of the scapula usually overlies the fifth intercostal space.

The spinous processes of all the vertebra from the seventh cervical to the fourth lumbar can usually be felt in the midline of the back (Fig. 5) 1912. With the patient bent forward in a sitting position the lumbar spinous processes become more widely separated and easier to identify. The first thoracic vertebra is usually the most prominent process, though the seventh cervical vertebra is equally prominent, or more so in some individuals. Flexion in the neck increases the prominence of these spinous processes. The spinous processes are normally counted downwards from the seventh cervical or upwards from the fourth lumbar, which can be recognized by its position at the same level of the highest part of the iliac crest. In the ordinary standing position the scapula overlies the first to seventh ribs. The root of the spine of the scapula can be used as an approximate guide to the third thoracic spinous process and is usually at about the same level as the suprasternal notch. The medial border of the scapula is somewhat obscured by the rhomboid muscle, and the distance of the medial border of the scapula from the spine shows great individual variation. The upper part of the scapula is deeply placed between the thick muscle mass, comprising supraspinatous and the trapezius. The clavicles anteriorly on either side form sinuous ridges at the junction of the thorax and neck. The suprasternal notch is bounded inferiorly by the upper border of the manubrium, and separates the lower attachments of the two sternomastoid muscles. A depression, the infraclavicular fossa is found below the middle third of the clavicle and provides access for cannulation of the subclavian vein.

The male nipple commonly lies opposite the fourth intercostal space, almost 10 cm (4 inches) from the midline, although its position varies considerably. The female breasts occupy an approximately circular area, which extends from the second rib to the sixth, and from the sternal margin to the axilla. The lower edge of the breast is marked by a pronounced, slightly curved groove, usually opposite the sixth rib in the midclavicular line. In girls younger than childbearing age, this line may be as high as the fifth intercostal space; in older or multiparous women it may be as low as the lower border of the seventh rib. The breasts are seldom exactly equal in size and are separated from each other by a cleft. There is an upward extension into the axilla, which obscures the lateral border of the pectoralis major muscle, where this enters the anterior wall of the axilla. Two-thirds of the breast overlies the pectoralis major muscle; the remaining one-third covers the serratus anterior and a small part of the external oblique and rectus abdominus muscles. The position of the female nipple varies greatly with the size, shape, and position of the breast. During pregnancy the glands greatly increase in size, whereas in old age they become atrophic.

Each thoracic spinal nerve emerges from the intervertebral foramen and divides almost immediately into the dorsal ramus and ventral ramus. The dorsal ramus of the thoracic nerve passes posteriorly through the erectus spinae muscle, the trapezius muscle, and other superficial muscles of the back to reach the superficial fascia. Here it divides into medial and lateral branches, which supply the skin. The ventral ramus of the thoracic nerve forms the intercostal nerve of that particular level. The white and grey rami communicantes connect the ganglia of the sympathetic trunk and the thoracic nerve of the same level near the origin of the ventral ramus. From the seventh to the eleventh thoracic vertebrae the ventral rami continue forward from the intercostal spaces into the anterior abdominal wall. Each intercostal nerve runs forward in the thoracic wall on the inner aspect of the internal intercostal muscle, lying inferior to the intercostal vein and artery. It gives off a collateral branch to the lower part of the intercostal space, as do the corresponding vein and artery. Each nerve has a lateral cutaneous branch at the lateral aspect of the thorax that pierces the overlying intercostal muscles to reach the subcutaneous tissues. The nerve then divides into an anterior (mammary) and a posterior branch. The intercostal nerve ends by becoming the anterior cutaneous nerve at the anterior end of the intercostal space.

The aorta, which lies on the left anterior aspect of the vertebral bodies, gives off pairs of posterior intercostal arteries. These pass forward in the upper part of the intercostal space between the intercostal vein above and intercostal nerve below. Anteriorly they anastomose with the intercostal branches of the internal mammary and musculophrenic arteries. Collateral branches run in the inferior part of the intercostal space. The right posterior intercostal arteries cross the anterior aspect and right side of the corresponding vertebral body as they pass to reach the right-sided intercostal space. The internal mammary arteries originate from the right and left subclavian arteries then descend through the thoracic inlet to pass along the inner aspect of the costal cartilages as far as the sixth costal cartilage, where the vessel branches into the superior epigastric, gastric, and musculophrenic arteries. The intercostal and internal mammary arteries are accompanied by the corresponding veins. The anterior intercostal veins end in the musculophrenic vein and the venae commicantes of the internal mammary artery. The posterior intercostal veins, apart from the first, join the azygos vein. On the left the lower veins join the hemiazygos venous system, whereas the second and third veins form the left superior intercostal vein which crosses the arch of the aorta and ends in the left innominate vein. The first intercostal veins on both right and left sides join the innominate vein.

The cavity of the thorax is completely divided into right and left lateral parts by a thick median partition, the mediastinum. The mediastinum is made up of a large number of important structures embedded in connective tissue extending from the sternum anteriorly to the vertebral column. The principal structures are the heart within the pericardium, the aorta and great vessels, the oesophagus and trachea, the vagus and phrenic nerves, the remains of the thymus gland, and numerous lymph nodes. The connective tissue ensheaths these structures but is both yielding and elastic, to accommodate the dilatation and contraction of the heart and blood vessels. Anatomically, the mediastinum is described in four parts: (a) the superior mediastinum above the plane passing from the lower border of the manubrium sterni to the lower border of the fourth thoracic vertebrae. The mediastinum below that plane is subdivided into (b) the middle mediastinum occupied by the pericardium and its contents with the phrenic nerves on either side, (c) the anterior mediastinum in front of the pericardium, and (d) the posterior mediastinum behind the pericardium and diaphragm (Fig. 6) 1913.

The lateral parts of the thoracic cavity are lined with a thin layer of pleura and are almost completely filled by the lung, which evaginates the pleural sac from the mediastinal side. Each lung lies freely within its pleural cavity, except along the medial surface where it is attached to the mediastinum by the root, consisting of the bronchus coming from the bifurcation of the trachea and the pulmonary artery and two pulmonary veins passing to and from the heart. When air is squeezed out of the lung, its bulk is not nearly sufficient to fill this space which it normally occupies. Under normal conditions the lung remains distended by atmospheric pressure, but when air is admitted into the pleural cavity the atmospheric pressure on the outer surface of the lung is made equal to that on the interior, so that the elasticity of the lung substance forces air out. The lung then shrinks to about one-third of its previous bulk. Similar shrinkage also occurs when blood or other types of fluid accumulate in the pleural sac.

The apex of each lung reaches as far superiorly as the vertebral end of the first rib and usually extends about 2.5 cm above the medial one-third of the clavicle (Figs. 2 and 5) 1909,1912. The anterior border of the right lung descends behind the sternoclavicular joint and almost reaches the midline at the sternal angle. It then continues downwards behind the sternum to the level of the sixth chrondrosternal junction. Here the inferior border curves laterally and inferiorly, crossing the sixth rib in the midclavicular line and the eighth rib in the mid-axillary line (Fig. 2) 1909. It then passes posteriorly towards the midline at the level of the spinous process of the tenth thoracic vertebrae (Fig. 5) 1912. These levels apply to the lung in expiration: on inspiration the levels for the inferior border are usually two ribs lower as the expanding lung descends into the costodiaphragmatic recess of the pleura.

The position of the anterior border of the left lung is similar to that of the right, though at the level of the fourth costal cartilage it deviates laterally in front of the heart, causing a cardiac notch in this border of the lung. The inferior border of the left lung is similar in position to that of the right, though it usually extends a little more inferiorly since the right lung is pushed up by the liver below the diaphragm on that side. The right lung is composed of three lobes, the upper, middle, and lower, separated by the oblique and horizontal fissures. The left lung ordinarily has only two lobes, the upper and lower, separated by the oblique fissure. There is no horizontal fissure in the left lung. Because neither lung extends as far inferiorly as the parietal pleura, some of the diaphragmatic parietal pleura remains in contact with the costal parietal pleura. This potential space, which may fill with air or fluid and whose size varies with the phases of respiration, is called the costodiaphragmatic recess. A similar, but much smaller area is present where the anterior border of the lung does not extend to the medial limit of the pleural cavity. This area is called the costomediastinal recess. The diaphragm separates the heart and right lung from the liver. The left lung is separated by the diaphragm from the stomach and spleen. Posteriorly, the lung extends from the level of the first thoracic vertebra as far inferiorly as the diaphragm, and the base of the lung is applied to the superior surface of the diaphragm. Because of the diaphragm's domed shape the highest point on the base of the right lung is at the level of the eighth to ninth thoracic vertebra; the highest point of the base of the left lung is about 1 cm lower (Fig. 6) 1913. From these high points, the bases of the two lungs follow the curves of the diaphragm to reach the lower levels previously described. The weight of a healthy adult right lung containing an average amount of blood is about 620 g, and that of the left is 570 g.

The heart is contained within a fibroserous sac, the pericardium. This also surrounds parts of the great vessels as they enter and leave the heart. The fibrous pericardium is conical, the apex being pierced by the aorta, the pulmonary trunk, and the superior vena cava. The base rests on the diaphragm and is fused with the central tendon in the median plane. The right posterior part of the pericardium is pierced by the inferior vena cava. The diaphragm separates the pericardium from the liver and from the fundus of the stomach.

The pericardium lies behind the body of the sternum and the cartilages of the second to sixth ribs, inclusive. Its anterior surface is separated from them by the lungs and pleurae, except in the median plane where the anterior surface of the fibrous sac is attached to the upper and lower parts of the body of the sternum. On the left side the pleura retreats from the sternum at the level of fifth costal cartilage. This is the area of the cardiac notch of the left lung. Each side wall of the pericardium is in relation to the mediastinal pleura. The median part of the posterior surface of the pericardium is in front of the descending aorta and oesophagus. The lateral part on each side posteriorly is supported by the pleura and lung. Within the fibrous pericardium, the serous pericardium is a closed and evaginated sac lining the fibrous sac and enveloping the heart and parts of the great vessels.

Details of intracardiac anatomy are only of direct relevance to the cardiac surgeon. Of great importance in trauma management are the relationships between cardiac structures and surface markings, which allow damage to intracardiac structures to be predicted in patients with penetrating chest wounds. The precise position of the heart is variable. The outline of the heart as projected on the anterior chest wall in different subjects during quiet respiration, may be classified into one of three general types: oblique, which is the most usual, horizontal, and vertical. These types tend to be related to general body build and the position of the diaphragm. The horizontal heart tends to occur in individuals with a high diaphragm and a short, broad trunk, whereas the vertical occurs in those with a low diaphragm and slender trunk. Surface markings also vary according to the position of the body, the phases of respiration, and with intrinsic movements of the heart. This variability clearly makes it impossible precisely to outline the heart in any individual.

In general terms, it may be stated that in the erect position the heart reaches a little to the right of the sternum and some 7.5 to 10 cm (3 to 4 inches) to the left of it. Two-thirds of its vertical height lies behind the sternum, reaching up to the sternal angle. Up to one-third may lie below the xiphisternal joint. The heart is 2 cm higher in the recumbent position than in the erect position. The upper border of the heart is concealed by the aorta and pulmonary trunk. Its position is marked on the surface by a line drawn from the lower border of the second left costal cartilage, 4 cm from the median plane to the upper border of the right third cartilage, 2.5 cm from the median plane. Since the pulmonary arteries run along the upper border of the heart, the right two-thirds of this line indicate the position of the right artery and the left third indicates the left artery. The right border of the heart is marked by a line slightly convex to the right, drawn from the upper border of the third right costal cartilage, 2.5 cm from the median plane, down to the sixth right cartilage, 1.25 cm from its junction with the sternum. The lower or diaphragmatic border of the heart is concave downwards, in correspondence with the upper convexity of the diaphragm on which it rests. There is a slight inclination downwards as the border is traced from right to left. This is marked by a line drawn from the sixth right costal cartilage near the sternum to the point opposite the apex in the fifth left intercostal space about 7.5 cm from the median plane. The left heart border is marked by a curved line drawn from a point opposite the apex to a point on the lower border of the second left costal cartilage, 4 cm from the median plane.

The precise level of the heart is influenced mainly by the level of the diaphragm. When the heart is situated at a low level the positions of the great arteries also differ, the arch of the aorta and pulmonary trunk being drawn downwards. When the heart is raised by upward movement of the diaphragm in expiration it becomes shorter in its vertical diameter and lies more transversely. When the diaphragm is lowered in inspiration the heart descends and appears narrower, assuming a more vertical position. The state of distension of the fundus of the stomach may affect the level of the dome of the diaphragm, and in turn modifies the position of the heart. When the fundus is greatly distended the heart tends to be pushed towards the right. Lateral displacement may occur in patients with scoliosis; displacement also occurs when pressure conditions in the pleural cavities become unequal, as in pneumothorax or haemothorax. The actual size of the heart changes during the cardiac cycle and is affected by intrathoracic pressure. When the pressure becomes relatively high at the end of the forceful expiration, the heart is, for a short period, diminished in size. The size of the heart relative to that of the thorax varies greatly in different subjects and it is therefore difficult to determine whether a slight pathological enlargement is present in a given subject. The size ratio of the heart to the thorax varies in normal subjects between one-third to one-half; this affords a rough guide to normal size. The heart in a child is relatively larger compared to the whole thorax than is the case in the adult.

The tricuspid valve lies behind the sternum, extending from the midline at the level of the fourth costal cartilage down towards the sixth right chrondrosternal junction. The mitral valve lies behind the left side of the sternum obliquely at the level of the fourth costal cartilage. The pulmonary and aortic valves lie close to the mitral valve, the pulmonary valve lying horizontally behind the inner end of the third left costal cartilage and adjoining part of the sternum (Fig. 8) 1915. The aortic valve is placed obliquely behind the left side of the sternum, opposite the third intercostal space. The right border of the heart begins behind the second costal cartilage with the superior vena cava, this joins the right atrium at the level of the third costal cartilage. The right atrium itself extends between 1 to 2 cm lateral to the sternum as far as the xiphisternal joint. The left border of the heart starts between the first and second costal cartilages with the aortic knuckle; beneath this is the left pulmonary artery and pulmonary trunk. The left atrial appendage lies below the left pulmonary artery between the second and third costal cartilages. The left ventricle then extends obliquely laterally from the second costal cartilage to the apex of the heart, which generally lies 9 cm from the midline behind the fifth left intercostal space. The site of the apex may vary in a normal person from 6 to 10 cm lateral to the midline and in a vertical direction ranges from the fourth intercostal space to the sixth costal cartilage. The diaphragmatic border of the heart consists principally of the right ventricle to within 2.5 cm of the apex.

The orifices of the coronary arteries are immediately above the level of the cusps of the aortic valve. The right coronary artery passes forward between the pulmonary trunk and the right atrial appendage and then runs downwards in the atrioventricular groove to the lower part of the right margin of the heart, around which it curves before proceeding along the diaphragmatic surface. During this course this vessel is protected by the body of the sternum on the diaphragmatic aspect of the heart; the posterior descending and left ventricular branches are vulnerable to penetrating injury passing upwards from the costal margin. The left coronary artery runs for a short distance towards the left behind the main pulmonary artery, and then passes forward between the pulmonary trunk and the left atrial appendage. The left main coronary artery divides into the circumflex branch, which curves backwards and downwards, and the left part of the atrioventricular groove reaching the lower border of the base of the heart, where its termination approximates to branches of the right coronary artery. Marginal branches of this vessel supply the posterior aspect of the heart. The left anterior descending coronary artery arises at the point where the left main stem appears between the pulmonary trunk and left atrial appendage. It then runs in the anterior interventricular groove towards the apex giving septal branches to the interventricular septum, and diagonal branches to the anterolateral aspect of the left ventricle. Both the left anterior descending and circumflex coronaries are vulnerable to penetrating wounds of the left chest.

Recent developments in imaging methods and interventional techniques have modified the radiological assessment and management of patients requiring thoracic surgery. However, the plain chest radiograph remains the initial investigation. In acutely ill or injured patients the radiograph is first evaluated for immediate life-threatening complications such as pneumothorax, haemothorax, rib fractures, and mediastinal air or widening indicative of aortic laceration. More detailed analysis of the radiograph may then be performed. Following stabilization of the patient's condition, further radiological investigations such as computed tomography or arteriography may be indicated.

The quality of the chest radiograph is dependent on the ability of the patient to co-operate with the procedure. Supine antero-posterior views may be misleading: in particular a pneumothorax may be missed. Whenever possible an erect or semi-erect view should be taken. If this is not possible a lateral decubitus view with the injured side uppermost, should be performed.

A knowledge of the normal anatomy of the chest and its variations is essential for the accurate interpretation of the chest radiograph, which should be viewed in a systematic manner. Scrutiny of the radiograph should be performed at the standard viewing distance and from afar (2 m). Fixed distance viewing is associated with an increased risk of perceptual error. At the standard viewing position, lesions with sharply defined borders are readily appreciated. However, with distance viewing, masses such as lung tumours with poorly defined margins are better perceived; in particular, subtle differences in lung density are more obvious.

It is important to remember that individuals with thoracic trauma may have other important chest problems unrelated to their injuries when reviewing the radiograph. An unsuspected lung cancer may be detected, or the superior mediastinum may be widened by an intrathoracic goitre. It is therefore important to perform a careful clinical examination of the chest, neck, and abdomen before interpreting the chest radiograph. Appropriate diagnoses can only be made in the light of combined physical and radiological findings.

A recommended sequence of inspection of the chest radiograph is as follows: chest wall, (the soft tissues and bony thoracic cage); the diaphragm and subdiaphragmatic area; the mediastinum; the trachea and major airways; the pleura; the lung fields.

Lesions in the bone or soft tissue of the chest wall may be misinterpreted as representing intrathoracic disease and may be the most important diagnostic feature. Surgical emphysema, (air tracking within the tissues planes and muscle bundles) may be more obvious than an underlying pneumothorax.

Two muscles are commonly visualized. The anterior axillary fold formed by the pectoral muscles is seen curving medially and inferiorly from the axilla to the rib cage. In muscular men, the pectoralis major may appear as a continuation of the anterior axillary fold, passing obliquely across both lungs. This muscle is responsible for the increased shadowing over the middle and upper part of the lung. When absent this will give rise to an area of hypertranslucency in that region of the lung, and a misleading impression of emphysema. In women, such absence is usually the result of a mastectomy, but it may also be a congenital abnormality (Poland syndrome). The sternomastoid muscle may be visualized as a density whose lateral margins parallel the spine as it runs down vertically from the lung apices over the medial third of the clavicle. This may cause confusion in the apices of the lung, where the muscle shadow may be misinterpreted as pleural thickening.

The presence and symmetry of breast shadows should be taken into account when assessing the density of the lower lung zones. The female breast typically extends from the second to the sixth rib. Prominent breast tissue in the male suggests gynaecomastia. The nipples are identified by their size (0.5 to 1 cm), shape, and location. The lateral border of the nipple is usually sharply defined, but the medial border is poorly defined.

Companion shadows are reflections of soft tissue visualized as 2 to 3 mm shadows which parallel the superior aspect of the clavicles and the inferior and inferolateral margins of the first and second ribs. Companion shadows may be mistaken for pleural thickening. In obese individuals the companion shadows may be wider due to an abundance of extrapleural fat. The frequency with which the companion shadows are seen varies. In normal individuals the companion shadows may be bilaterally symmetrical, asymmetrical, or invisible on both sides (Fig. 11) 1920.

The thoracic cage should appear symmetrical. The general shape, direction, and spacing of the ribs should be noted. The ribs run roughly parallel, the anterior ends of the ribs lying at a lower level than the posterior ends. The posterior portions of the ribs are directed downward and laterally whereas the anterior parts are directed downward and medially. Deformities of the dorsal spine distort the contour of the chest and may cause considerable difficulty in interpretation of the chest radiograph. In kyphosis, the posterior parts of the rib are crowded together. In scoliosis the ribs are more vertically inclined on the concave side of the spine.

The first rib is identified by its broader anterior end and by its articulation with the first thoracic vertebra. The latter can be distinguished from the seventh cervical vertebra by the orientation of its transverse processes which are inclined upwards and laterally, whereas those of the seventh cervical vertebra point downwards and laterally (Fig. 16) 1925. By following the ribs backwards the number of the thoracic vertebra can be ascertained, remembering that most of the ribs articulate with the upper parts of the bodies of the corresponding vertebra. The superior and inferior contours of the upper ribs are well defined. The inferior cortex of the ribs is less distinct in the mid- and lower thoracic region due to visualization of the vascular sulci. Focal superficial indentation of the inferior margin of the posterior ribs adjacent to the tubercle is a normal appearance. This indentation is distinct from pathological rib notching seen in coarctation of the aorta, which is characteristically situated more laterally near the midclavicular line. Calcification of the costal cartilages is common and of no clinical significance. Calcified costal cartilages may cause confusion, however, particularly in the line of the first rib where they may resemble intrapulmonary lesions. The first costal cartilage normally calcifies first, in the second decade, followed by calcification of the lower costal cartilages in middle age. The pattern of calcification is different between the sexes. In men the upper and lower borders of the cartilages calcify first followed by central calcification; this produces a marginal pattern. In women central calcification occurs as bands or as discrete nodules.

Congenital anomalies of the ribs are uncommon. Cervical ribs occur in 1.5 per cent of normal subjects and are usually bilateral but asymmetrical (Fig. 12(a)) 1921. Occasionally ribs may be absent or hypoplastic. Bifid or local fusion of ribs should be identified to avoid misinterpretation as representing a lung opacity. Supernumerary ribs such as an intrathoracic or pelvic rib usually occur on the right and are extremely rare (Fig. 13) 1922. Rib abnormalities may be associated with vertebral anomalies.

In injured patients the pattern of rib fractures should be noted. Subtle rib fractures are unlikely to be of clinical importance. Multiple bilateral rib fractures indicate a flail chest, while mediastinal injury should be suspected if the upper three ribs are fractured. Abdominal visceral injury should be considered if the lower three ribs are injured.

The clavicles are frequently fractured by thoracic trauma and may in turn lacerate the subclavian vessels or brachial plexus. The rhomboid fossa is an irregular notch on the inferior surface of the lateral aspect of the clavicle and is seen in 0.6 per cent of normal individuals (Fig. 14) 1923. The fossa gives rise to the costoclavicular ligament that binds the clavicle to the first rib: this is bilateral in 33 per cent of individuals in whom it is present. A foramen for the middle supraclavicular nerve may occasionally be seen either unilaterally or bilaterally. The conoid tubercle may be visualized on the inferior lateral aspect of the clavicle at the sight of insertion of the costoclavicular ligament.

Minor congenital anomalies of the manubrium are frequent. A wide manubrium sterni can project over the lung fields and may cause confusion, resembling a mediastinal abnormality. A depressed sternum (pectus excavatum) is common: its recognition is important to avoid erroneous interpretation of the mediastinum and lung fields. It can be confidently diagnosed on the posteroanterior radiograph by a number of features (Fig. 15) 1924. Typically, the heart appears shifted to the left with a straight left heart border and increased cardiothoracic ratio. There is loss of clarity of the right heart border and an ill-defined opacity is visualized at the right cardiophrenic area due to the soft tissues of the anterior chest wall, as they slope backwards and forwards to reach the sternum. The main pulmonary artery is prominent. The anterior ribs slope steeply downwards and there is undue clarity of the lower dorsal spine. The lateral radiograph will demonstrate the degree of sternal depression. In pectus carinatum the anteriorly bowed sternum may be a developmental anomaly, associated with congenital heart disease, or acquired due to asthma.

On the frontal radiograph the sternum is obscured by the thoracic vertebrae and mediastinal shadows. Clinically, a sternal fracture is suggested by bruising, swelling, and tenderness over the sternum; it may be confirmed radiologically on a detailed lateral view. The scapula on a posteroanterior radiograph should not obscure the lung fields, but on an anteroposterior and lateral view, they project over the lung fields and their normal appearance must be recognized to avoid misinterpretation. This is particularly important when the scapulae are not symmetrically positioned; in this situation one lung field may appear less illuminated than the other, leading to the false diagnosis of a pathological process. By following the contour of the scapula and identifying the medial borders and inferior angles, this error should be avoided. A fracture of the scapula is a hallmark of a very severe blow to the chest.

The hemidiaphragm is seen on the frontal radiograph as a smooth curve convex upwards with the highest point near the middle of the lung fields. Only the upper surface of the hemidiaphragm is seen because of contrast with the air-filled lungs; the undersurface blends with the opaque abdominal structures. The medial half of the left hemidiaphragm is not seen due to the heart lying immediately above and preventing the air-filled lung providing radiographic contrast. The lateral costophrenic recesses are usually clearly defined and form an acute angle. The recess is the first area to be obscured when the pleural cavity fills with blood or fluid. Exaggeration of the recess may occur with a small pneumothorax and may be the most obvious sign of a pneumothorax being present. The cardiophrenic angles are usually less well defined and less acute.

The height of the hemidiaphragms varies with the phase of respiration. On full inspiration the diaphragm lies between the fifth and seventh anterior rib space, at the level of the tenth thoracic vertebra. In 3 per cent of normal individuals it may be as high as the fourth interspace or as low as the seventh rib; it is rarely lower. If the right dome lies at the level of the seventh rib, it can only be considered normal if it has retained a curved contour and moves more than 3 cm on expiration. The height of the diaphragm is higher in women, individuals over 40 years, and in infants. Differences in height are frequently associated with body build, the diaphragm being lower in the slender type of individual and higher in those of a stocky build. In obese patients the diaphragm is displaced upwards by an increase in abdominal fat; displacement also occurs when there is excess gas in the intestinal tract. The diaphragm is commonly 3 cm higher in the supine position than in the erect position.

The right hemidiaphragm is typically higher than the left by up to 3 cm. In 10 per cent of individuals the left hemidiaphragm is at the same level as or higher than the right hemidiaphragm. Gaseous distension of the stomach or splenic flexure elevates the left hemidiaphragm. The height of the hemidiaphragm is determined by the systemic ventricle. The apex of the heart lies on the same side as the lowest hemidiaphragm. In the decubitus position the diaphragm is highest on the dependent side.

The range of diaphragmatic excursion is variable. On quiet respiration the range of movement is about 1 cm, while in deep respiration the excursion may be between 3 and 10 cm. In 60 per cent of individuals the right hemidiaphragm moves to a greater extent than the left, and in 10 per cent of individuals the left moves more than the right. However, the difference in excursion between the hemidiaphragms is less than 1.5 cm.

The curve of the hemidiaphragm may be assessed visually or by drawing a line across the chest, from one costophrenic recess to the other, and then a second line vertical to this from the highest point of the dome. In normal individuals it will measure 1.5 cm or more. If the vertical line measures 1 cm or less it indicates flattening of the dome, as in emphysema. Loss of definition of the diaphragmatic contour indicates poorly aerated lung adjacent to the diaphragmatic pleura and is a sign of underlying lung disease. If the clarity of the diaphragm is preserved, opacification of the lower zone is probably due to pathology in the overlying thoracic wall soft tissues. Translucencies beneath the hemidiaphragms should be identified as lying within an abdominal viscus, otherwise these lucencies represent free intraperitoneal air. If no gas is seen within the stomach below the diaphragm, a hiatus hernia or achalasia should be considered. Chilaiditi's syndrome is the interposition of the colon in front of and above the liver. It is common in children and the elderly and should not be mistaken for subdiaphragmatic free air.

Physiological variations in the diaphragmatic contour may occur. In scalloping of the diaphragm, the normal smooth convex contour is replaced by two smooth arcuate elevations. This is of no clinical significance and is usually confined to the right side. Diaphragmatic muscle slips may be visualized, originating from the lateral and posterolateral ribs as short curves, concave upwards along the lateral part of both hemidiaphragms. These are produced by exceptionally low descent of the diaphragm during inspiration and may occur in healthy young men. However, it is more commonly seen in association with severe pulmonary overinflation, as in asthma or emphysema. Muscle slips are prominent at full inspiration but disappear on expiration.

Eventration is characterized by congenital fibrous replacement of the diaphragm (Fig. 16) 1925. It is usually left-sided, but partial eventration of the anteromedial segment of the right hemidiaphragm may also occur, producing a hump. This should not be misinterpreted as a mass lesion or middle lobe consolidation. A fat pad in the anterior cardiophrenic recess may give a similar appearance. Accumulation of fat in the cardiophrenic recess is common and may occur in obese and non-obese individuals. On the left, the fat pad may simulate cardiac enlargement, but the fat is identified as having a density less than that of the adjacent heart.

Gastric herniation through the oesophageal hiatus is extremely common (Fig. 17) 1926. Herniation through the foramina of Morgagni and Bochdalek also occurs. The Morgagni hernia occurs anteriorly; liver, omentum, or bowel (especially colon) may herniate between the sternal and costal origins of the diaphragm (Fig. 17) 1926. The Bochdalek hernia is due to a posterior deficiency of the diaphragm and may contain retroperitoneal fat, part of a kidney, or the spleen.

Diaphragmatic rupture can be difficult to diagnose, as the plain radiographic findings are non-specific, particularly in ventilated patients. However, it is an important diagnosis to consider, since early recognition may prevent the potentially lethal complication of bowel strangulation. Diaphragmatic rupture seems to be more common on the left side. Partial rupture may go unrecognized and may remain asymptomatic; alternatively the patient may present later with bowel obstruction. On the right, the appearance is of an apparently elevated diaphragm due to herniation of the liver through the defect. However, there is usually an associated haemothorax above the liver shadow and loss of the costophrenic angle (see Section 35.12 273). On the left, a ruptured diaphragm is suspected when stomach, colon, or small bowel prolapses into the chest and corresponding shadows are seen above the usual level. If necessary, this may be confirmed following oral administration of contrast medium. The shadow of an intact or ruptured spleen may be apparent above the level of the diaphragm and a haemothorax usually coexists. In patients receiving positive pressure ventilation, the supine radiograph may not reveal pathological elevation of the diaphragm. Ultrasound may be helpful, localizing the hemidiaphragm and demonstrating the presence of an intrathoracic abdominal viscus. A subpulmonic haematoma may mimic an elevated hemidiaphragm. However, the upper border of the opacity is less convex, the apex is situated more laterally, and the contour is less clearly defined compared to the hemidiaphragm. A decubitus film will confirm the presence of pleural fluid.

The infradiaphragmatic area has special importance in chest injuries, since the rib cage encompasses a substantial part of the upper abdomen and thoracic trauma frequently involves the abdominal viscera. Splenic lacerations are particularly common after blunt trauma. Fractures of the lower three ribs on the left side should alert one to the possibility of splenic trauma. The spleen lies to the left of the stomach, and when ruptured a perisplenic haematoma may occupy a considerable area under the diaphragm, displacing the gastric air bubble medially. The stomach is identified by an air bubble in the fundus which is usually larger than the bubble within the splenic flexure. The splenic flexure may contain faeces, thus identifying the viscus as colon. On the right side the subdiaphragmatic area is occupied by the opaque shadow of the liver. The diaphragm separates from the liver only when there has been rupture of a gas-filled viscus and this may readily be identified on an erect chest radiograph.

The mediastinum lies in the midplane of the thorax between the lungs, from which it is separated by the visceral and parietal pleura. The thoracic inlet lies superiorly, and the diaphragm inferiorly. The conventional plain radiograph provides suboptimal demonstration of the mediastinum. However, with the use of higher voltage films (125–150 kV range) the mediastinal structures are more clearly visualized. The silhouette should be sharply defined. On the frontal radiograph the right mediastinal border is formed from above down by the right innominate vein, the superior vena cava, and the right atrium of the heart. When enlarged, the left atrium may form a double contour to the right heart border. The left atrium is situated at a higher level than the right atrium, so that the lower border of the heart consists of right atrium and inferior vena cava. The left border is formed by the left subclavian artery, the arch of the aorta, forming the so-called aortic knuckle, the pulmonary artery or its left branch, the left atrial appendage, and the left ventricle. The pericardium itself is not normally visualized, but if air fills the pericardium after major airways injury the thin pericardial sac can be clearly seen. The appearance of the mediastinum has great important in chest injuries, since damage to the major vessels will produce mediastinal widening due to haematoma. This is usually apparent on the plain radiograph (see Section 35.12 273).

It is frequently difficult to determine whether a centrally situated mass lesion lies in the mediastinum, the pleural space, or within the lung parenchyma. Three radiological features help to confirm a mediastinal lesion. A mediastinal mass will usually have a smooth, sharply defined contour. While a pulmonary mass may also have this appearance, a mediastinal mass will rarely have a poorly defined border. A mediastinal mass tends to form obtuse angles between its margins and contiguous lung, compared with the acute angle formed by a pulmonary lesion. Finally, a mass lesion that is in contact with or displaces mediastinal structures is likely to be mediastinal in origin. In convention, a mediastinal mass is classified by its location, in the anterior, middle, or posterior mediastinal compartment. This is a descriptive definition and is not related to anatomical boundaries.

The thoracic inlet is the region at the cervicothoracic junction lying in a transverse plane at the level of the first rib. It is higher posteriorly than anteriorly. The mediastinal fascia extends in continuity across the thoracic inlet where it merges with the cervical fascia. Thus the neck and mediastinum communicate freely, allowing the spread of air or disease through the cervical-thoracic continuum.

The aortopulmonary window lies with the aortic arch above and the left pulmonary artery below. It contains the ligamentum arteriosus, ductus nodes, left recurrent laryngeal nerve, and fat. The aortopulmonary line is a sharply defined interface that extends vertically from the aortic arch to the level of the left main bronchus. Lymphadenopathy produces a prominent convex bulge in this region. End-on visualization of the left superior intercostal vein may produce a protuberance along the aortic knuckle which has been termed the aortic nipple. The nipple should not be mistaken for lymphadenopathy.

The superior mediastinal shadow is caused by the great systemic vessels. The heart hangs from these vessels, known as the vascular pedicle. The width of the pedicle is measured from the point at which the superior vena cava crosses the right main bronchus to a line drawn perpendicular to a point at which the left subclavian artery arises from the aortic arch. In the majority of normal individuals the pedicle measures less than 58 mm. In cardiogenic oedema, the vascular pedicle is increased in width while in non-cardiogenic oedema the pedicle is of normal or reduced width.

The heart lies in the midline, projecting to the left. There is a wide variation in normal cardiac size, which depends on body build. Traditionally, cardiac size is assessed by comparing the transverse cardiac diameter to the transverse diameter of the chest (cardiothoracic ratio). The value of 50 per cent is widely accepted as the upper limit of normal for this ratio. However, this is exceeded in 10 per cent of normal subjects. Individuals with a small cardiac diameter may undergo a considerable increase in cardiac size without exceeding a cardiothoracic ratio of 50 per cent. It has been suggested that an absolute value of 15.5 cm for the upper limit of normal for cardiac diameter is a more appropriate measurement. Serial radiographs are useful for monitoring cardiac size. The size and contour of the heart varies with the influence of systole and diastole, but on average, during the cardiac cycle, the cardiac diameter does not vary by more than 2 cm. The cardiac configuration is also affected by the height of the diaphragm. The greater the degree of pulmonary inflation the lower the position of the diaphragm and the longer and narrower is the cardiovascular silhouette. A poor respiratory effort will therefore result in a short squat configuration to the heart. The heart will also appear broader when the subject is supine compared to the erect position. In infants and young children, a cardiothoracic ratio of 60 per cent is accepted as normal.

Although the shape of the heart varies between long narrow, and vertical to a more rounded horizontal shape, the left heart border between the aortic knuckle and the left ventricle should maintain a concave contour. In cardiac tamponade when the pericardium is filled with blood clot, this concavity is lost and the heart adopts a globular shape. Since the pericardium is relatively non-distensible the overall size of the cardiac shadow may not perceptively increase in tamponade. Pericardial fat pads may be visualized on the frontal and lateral radiographs. These are more commonly seen on the left; the configuration is variable and may change shape with respiration (Fig. 18) 1927.

Rupture of the aorta follows high speed deceleration trauma and is becoming increasingly more frequent. Approximately 95 per cent of ruptures occur in the region of the aortic isthmus, with development of a mediastinal haematoma. The radiological features of aortic injury are discussed in Section 35.12 273. The mediastinal haematoma may remain localized or may rupture into the left pleural cavity.

Air may enter the mediastinum, via the deep fascial planes of the neck or rarely by dissection from the retroperitoneal space. Pneumomediastinum is a highly significant finding. Its cause must be identified, as prompt treatment of perforation of trachea, bronchi, or oesophagus may be life-saving. Pneumothorax may occur secondary to pneumomediastinum but pneumothorax never causes pneumomediastinum. The signs of a pneumomediastinum are linear streaks of gas within the mediastinum, and displacement of the mediastinal pleura of the heart and other mediastinal structures. Retrosternal gas collections and visualization of the central portion of the diaphragm due to extrapleural dissection along the diaphragm, may occur. Pneumopericardium is rare and may be confused with pneumomediastinum. Pneumopericardium may be identified as air within the pericardium moves freely away from the dependent part of the sac, while mediastinal air shows little movement when the position of the patient is changed.

The air column of the trachea, major bronchi, and bronchus intermedius should be visible on the frontal radiograph. The trachea extends from the cricoid to the carina and is approximately 11 cm long. It is a midline structure but deviates slightly to the right as it passes the aortic knuckle. Tracheal deviation increases with age as the aorta enlarges. The coronal diameter of the trachea is 1.5 to 2 cm and its sagittal diameter is 2 cm. A coronal diameter greater than 2.5 cm or less than 1.3 cm in men and 1.0 cm in women is abnormal. The cross-sectional shape of the normal trachea is variable but it is most commonly round or oval. The trachea is bordered on its right lateral wall by pleura: this contains a variable quantity of fat forming the right paratracheal stripe. This stripe is visualized on the majority of radiographs and has a maximum normal width of 4 mm. Loss of the outer margin of the right paratracheal stripe suggests lymphadenopathy. The left paratracheal stripe is seldom seen.

Displacement of the trachea is an important radiological sign. However, in the young, the trachea and mediastinum are very mobile and often show a marked deviation from the midline, particularly on expiration. The lumen of the trachea should also be noted as the trachea may be narrowed, or opacified by a tumour or foreign body. The trachea divides into the two main major bronchi at the carina, which lies at the level of the fourth to fifth thoracic vertebra. The subcarinal angle shows a wide variation in normal individuals but on average measures 70°. The course of the right main bronchus is more vertical than is that of the left main bronchus in adults, which accounts for aspiration occurring more commonly on the right. In children, there is bronchial symmetry and the incidence of right- and left-sided aspiration of a foreign body is therefore similar. The transverse diameter of the right main bronchus is greater than that of the left. The left main bronchus is longer than the right. The right upper lobe bronchus arises more proximally than the left upper lobe bronchus and divides into three branches, the anterior, posterior, and apical segments. The intermediate bronchus continues distally for 3 to 4 cm, from the take-off of the right upper lobe bronchus and then bifurcates into the middle and lower lobe bronchus. The right middle lobe bronchus bifurcates into the medial and lateral segments.

The left upper lobe bronchus either bifurcates or trifurcates: the former is more usual. The superior branch divides into the apical posterior and anterior segments, while the inferior branch is the lingula bronchus, which is roughly analogous to the right middle lobe bronchus and divides into the superior and inferior segments. The left lower lobe bronchus divides in similar fashion to the right lower lobe bronchus. There is one exception: the anterior and medial portions of the lobe are supplied by a single anteromedial bronchus.

The pleura, consisting of the visceral and parietal layers, is not normally visible over the lung apices, mediastinum, or diaphragm. Slight pleural thickening (1 to 2 mm) over the convexity of the lungs can be visualized because of the greater density of the contiguous ribs. Even moderate thickening over the mediastinal and diaphragmatic surfaces cannot be recognized as these adjacent structures have a similar radiographic density. Focal pleural thickening, as is seen in mesothelioma, can be identified by the alteration of the smooth contour of these surfaces.

The interlobar fissures comprising two layers of visceral pleura form the contact surfaces between the pulmonary lobes. They are of hairline thickness and are visible because of the presence of air-containing lung on either side of the fissure. These fissures are only visible on the radiograph when the X-ray beam passes tangentially along their surface. All fissures have an undulating course and they are therefore never visualized in their entirety. The identification of the fissures is important in the assessment of lobar disease. Their depth varies from complete separation of the lobes to a superficial slit. The completeness of this separation determines the ease of spread of disease between the lobes and the relative ease of lung resection. The fissures should be assessed for position, configuration, and thickness.

The oblique fissure separates the upper (and middle) lobes from the lower lobes. It begins at the level of the fifth thoracic vertebra and extends obliquely downward and forward running parallel with the sixth rib and ending at the diaphragm a few centimetres behind the sternum. The left oblique fissure has a more vertical course and lies posterior to the right fissure. The fissure is propeller shaped, with the upper half facing forwards and outwards and the lower half facing forward and slightly medially. A triangular opacity at the lower end of the fissure with its base contiguous with the diaphragm and its apex tapering into the fissure is due to fat.

This fissure separates the anterior segment of the right upper lobe from the middle lobe and lies roughly horizontal at the level of the fourth rib anteriorly. The contour is variable and minimal displacement should not be regarded as significant. The fissure rarely reaches the mediastinum. Its medial extent is to the lateral margin of the interlobar pulmonary artery. A fissure line that projects medial to this point represents the downward projection of the oblique fissure, providing evidence of volume loss in the right lower lobe. On the frontal radiograph the horizontal fissure is seen in its entirety in 6 per cent of individuals and is visualized in part in 66 per cent.

A left minor fissure, separating the anterior segment of the upper lobe from the lingula, is an uncommon finding. On the lateral radiograph the left minor fissure is superior to the right minor fissure (Fig. 14) 1923.

Accessory fissures between segments are common and their recognition is important to avoid misinterpretation of the radiograph and to allow identification of the segment they subtend.

An azygos lobe is an uncommon anomaly, occurring in 0.4 to 1 per cent of individuals (Fig. 21) 1930. It is created by the downward invagination of the azygos vein through the apical portion of the right upper lobe. The azygos fissure is composed of a double layer of visceral and parietal pleura. The space between the layers of pleura is in communication with the general pleura cavity and may therefore contain air or fluid if a pneumothorax or pleural effusion is present. The azygos lobe should not be misinterpreted as right upper lobe collapse.

This fissure separates the medial basal segment from the remainder of the lower lobe, forming a retrocardiac lobe. This fissure is visualized in 30 per cent of chest radiographs.

This fissure separates the superior segment from the basal segments of the lower lobe and is more commonly visualized on the right.

The lungs should be reviewed individually and in comparison, matching zone by zone from apex to base. Before assessing the lung density the patient's position should be noted. Rotation of the patient is identified by observing the distance between the medial end of the clavicle and the adjacent margin of the vertebral body. It should be symmetrical in a correctly positioned patient. Rotation will alter the density between the lungs, the side closer to the film appearing blacker. Any discrepancy in density between the lungs in a patient who is not rotated must be regarded as pathological. Serial radiographs may show a variation in lung density, in the same subject depending upon the depth of respiration: the greater the degree of lung inflation, the more translucent the lungs appear.

The normal linear markings are composed of the pulmonary blood vessels and the fissures. An alteration in vasculature may be local or generalized, and should be interpreted in conjunction with an assessment of the hilar pulmonary vessels and the degree of lung inflation. Abnormal linear shadows may be identified from their compartment of origin, namely, the lung parenchyma, the interstitium, the bronchovascular bundles, or the pleura. Although it may be difficult to distinguish between parenchymal and interstitial origin, this method of evaluation is a useful aid to interpretation.

The silhouette sign is useful for identifying and localizing pulmonary disease. The cardiac and diaphragmatic outline is entirely dependent on adjacent air-filled lung for its visualization. If the lung opacifies the outline will be obscured. Any intrathoracic abnormality that obliterates the contour of the heart or diaphragm is in anatomical continuity with these surfaces. Loss of the right heart border implies middle lobe disease or, on the left, disease of the lingula. A collapsed and opaque lower lobe, lying posteriorly, will not efface the cardiac border but will obscure the adjacent diaphragmatic contour.

The pulmonary vessels should be examined from the periphery towards the hilum. This method prevents the more prominent hilar vessels from diverting one's attention from the less conspicuous vessels of the periphery. The vessels in comparable zones of the lung should be of similar size and number. The vessels should be clearly defined: marginal haziness indicates an abnormality of the perivascular connective tissue or lung parenchyma. The vessels in the upper zone of the lungs are smaller than are those of the lower zones at equivalent levels, because of preferential perfusion of the lung bases due to gravity. In the outer periphery (1 to 2 cm) of the lung, vessels are no longer visible. The pulmonary arterial system is invariably related to the bronchial tree; they branch together as they run in the bronchovascular bundles. The pulmonary veins lie within the interlobular septa and are thus separate from the bronchoarterial pathways. These vessels terminate medially into two superior and two inferior pulmonary veins. In the upper zones the veins lie lateral to the arteries, while in the lower zones they have a more horizontal course. The apical segmental upper lobe bronchus may be visualized end-on, at the hilum. Comparison with an adjacent opaque artery is a useful assessment of the calibre of the pulmonary circulation: normally the artery is of a similar calibre to the bronchus. Redistribution of blood flow resulting in the upper lobe vessels being larger than the lower lobe vessels is a well recognized sign of raised pulmonary venous pressure.

Evaluation of the hilum involves an assessment of its size, position, and density. A pathological process involving the hilum causes the hilum to appear too large, too small, dense, or in an abnormal position. The hilar shadows are formed by the pulmonary arteries and veins: bronchi and normal lymph nodes do not contribute to the hilar density. The upper portion of the hilar shadow comprises the upper lobe pulmonary veins, as well as the arteries. The lower portion of the hilum consists of the basal pulmonary arteries. The inferior pulmonary veins do not contribute the hilar shadow but are visualized end-on, creating a nodular opacity postero-inferiorly to the hilum. This pseudotumour appearance can be distinguished from a mass lesion by identifying vessels converging towards the opacity, the venous confluence.

The right pulmonary artery is an intramediastinal structure, dividing at the pericardium into the ascending and descending branches. The right main pulmonary artery does not form part of the hilar shadow. The left pulmonary artery arches above and behind the left main bronchus. The left hilum is therefore at a higher level than the right (by 0.5 to 1.5 cm): in 3 per cent of individuals the hilum may be at the same level but the right hilum is never higher than the left in normal subjects. The position of the hilum can be assessed from the hilar point. This is the level at which the superior pulmonary vein meets the shadow of the basal artery. On the right the hilar point is opposite the horizontal fissure (Fig. 23) 1932.

Hilar size is initially assessed by comparison of one hilum with that of the opposite side. The size range is quite variable between individuals, but in normal individuals they should be of similar size. A reliable assessment of hilar size can be made from measurement of the diameter of the right lower lobe pulmonary artery. This vessel, when measured at its widest point, should not exceed 16 mm in men and 15 mm in women. When wider than this pulmonary hypertension should be considered. The left lower lobe artery is normally poorly visualized, and assessment of its size is therefore difficult. The diameter of this vessel is smaller than the right by 1 to 2 mm, as blood flow is greater to the larger right lung. The inferior portion of the right hilum may have a rounded configuration rather than the more usual tubular appearance. This is due to the visualization, end-on, of a posteriorly directed right pulmonary artery, and is a normal anatomical variant. It may simulate a hilar mass.

Many congenital and acquired conditions cause abnormality in hilar size. The most common cause for a small hilar shadow is lobar collapse, particularly of the lower lobe. The hilum appears smaller as the lower lobe artery cannot be visualized when surrounded by airless lung: only pulmonary vessels supplied by the aerated lung are radiographically visible. In Macleod's syndrome, a small hilum is associated with peripheral hypovascularity and increased transradiancy of the whole or part of one lung (Fig. 24) 1933. It has been attributed to an obliterative bronchiolitis occurring in early childhood, and it is usually discovered as a chance radiographic finding in an asymptomatic individual.

A prominent hilum may be due to enlargement of the pulmonary artery or to a hilar mass. When the pulmonary vessels seem to arise directly from the hilar shadow, the prominent hilum is due to an enlarged pulmonary artery. However, if the pulmonary arteries arise medial to the lateral aspect of the hilar shadow then the enlargement is caused by an extravascular mass. The ‘hilum overlay’ sign allows differentiation of a mediastinal mass from a prominent cardiac silhouette. The first branching of the pulmonary artery arises lateral to the cardiac shadow or just within its outer aspect in the majority of normal individuals. In the presence of an anterior mediastinal mass, the hilum is projected medial to the lateral border of the mass and the first bifurcation of the pulmonary artery will lie more than 1 cm medial to the edge of this shadow. In cardiomegaly the hilum is displaced laterally.

The density and contour of the hilar shadows should be compared. Normally the lateral aspect of the hilum has a concave contour: this is more clearly visualized on the right. Hilar lymphadenopathy causes enlargement of the hilum and the development of a lobulated contour.

In lobar collapse, there is alteration of the hilar arterial and bronchial anatomy. With upper lobe collapse the vessels disappear, but the hilum appears to be elevated because the interlobar and lower lobe pulmonary artery remains visible. The ipsilateral main bronchus has a horizontal orientation and there is outward displacement of the bronchus intermedius on the right and the lower lobe bronchus on the left. In lower lobe collapse the hilum appears small rather than depressed. If the lower lobe pulmonary artery is visible, an opacity in the lower lobe cannot be due to collapse. The ipsilateral bronchial tree is swung medially and has a vertical orientation. The opacity of collapsed lung appears as a uniform whiteness, except in left upper lobe collapse. On the frontal radiograph, the collapsed left upper lobe appears as an area of opacification at the hilar region which fades to a more normal lucency lateral to the hilar region. As a lobe collapses, compensatory over-inflation of the adjacent lung occurs. The over-inflated lung appears blacker than usual with a decrease of vascular markings throughout the lung field. In upper lobe collapse an over-inflated superior segment of the lower lobe may insinuate between the mediastinum and the medial aspect of the collapsed lobe to produce paramediastinal lucency. This is more commonly seen on the left.

The lateral view is often of value in the assessment of thoracic trauma, particularly to visualize the thoracic vertebra, to define loculated collections of blood or fluid, or to locate penetrating foreign bodies. The chief landmarks on the lateral radiograph are: the bone and soft tissue shadows; the diaphragm; the mediastinum shadows; and the lung fields.

A fractured sternum can only be clearly defined on the lateral view. This may also disclose an associated haematoma in the anterior mediastinum that will be visualized on the posteroanterior view as mediastinal widening. The anteroposterior view may be interpreted as suggesting aortic injury. In children the manubrium and the four segments of the body of the sternum can be seen separately.

The thoracic spine is well visualized on the lateral radiograph. The alignment and vertebral body height should be noted. The spine is rigid and supported by ribs, consequently traumatic forces must be considerable to cause fractures and dislocation. Dislocations rarely present without fractures. The density of the thoracic spine on the lateral view normally becomes increasingly translucent from above downwards until the diaphragm is reached. Pleural or pulmonary pathology will alter this progressive increase in translucency and the whole length of the spine may appear equally radio-opaque. This observation may be the only evidence of a posteriorly positioned lesion which is hidden behind the heart on the frontal radiograph. A haemothorax may make it difficult to visualize the thoracic spine.

On the true lateral radiograph, a vertical opacity, the retrosternal stripe, may be visualized. It measures up to 3 mm and is composed of mediastinal fat. The parasternal stripe represents lung abutting on the anterior chest wall to the right or left, and has a wavy contour, due to the indentation of the surface of the lungs by costal cartilages. The contour may be lobulated due to anterior rib fractures.

The head of the humerus and glenoid may overlie the posterior aspect of the chest. The vertical border of each scapula is seen running downwards from the inferior margin of the glenoid which is directly behind the tracheal translucency.

The domes of the diaphragm pass in a superior convex curve from the sternum backwards to the vertebra. The highest level is usually at the mid-axillary line and the lowest is the posterior costophrenic recess. Localized pulmonary or pleural disease adjacent to the posterior aspect of the diaphragm, may not be recognized on the frontal radiograph but will be seen on the lateral view. Localization of the disease, therefore, requires identification of each hemidiaphragm on the lateral radiograph. The distinction is made by the following findings: the right dome is usually higher than the left and is visualized passing anteriorly through the heart; the left dome is obscured anteriorly by the heart and has the gastric air bubble lying beneath it; the inferior vena cava causes effacement of the right hemidiaphragm; visualization of the oblique fissures allows identification of the ipsilateral hemidiaphragm; the hemidiaphragm nearest the film is related to the least magnified posterior ribs.

The trachea is seen as a tubular air-containing structure extending from the neck to the level of its bifurcation at the sixth thoracic vertebra. The carina is rarely identified on the lateral radiograph. The position of the trachea should be noted and the posterior tracheal stripe representing the posterior tracheal wall and surrounding soft tissue should measure between 2 and 3 mm. The tracheo-oesophageal stripe formed by the posterior tracheal wall, the anterior oesophageal wall (identified when air is contained within the oesophagus), and surrounding mediastinal soft tissue should not measure more than 5 mm. The end-on orifices of the right and left upper lobe bronchi can be identified on the majority of radiographs: each bronchus lies in the plane of the trachea. The demonstration of an abnormal position of the upper lobe bronchi, either anterior or posterior to the plane of trachea, may be an important finding. In upper lobe collapse the bronchi may be displaced forward, while in lower lobe collapse the bronchi are displaced posteriorly. Posterior displacement of the left upper lobe bronchus may be the only sign of left atrial enlargement. The right upper lobe bronchus projects above the left bronchus. The left upper lobe bronchus is readily visualized as it is surrounded by vascular structures. The right upper lobe bronchus is rarely as well defined as it is surrounded primarily by lung. If the bronchial lumen is clearly visualized on the right the airway is likely to be surrounded by soft tissue due to lymphadenopathy. The posterior wall of the right main and bronchus intermedius is visualized extending in continuity with the posterior tracheal stripe. Anterior bronchial walls are rarely well seen. The posterior wall of the left main bronchus is seen in less than 50 per cent of patients. The walls of the central bronchi should be well defined and measure less than 3 mm in thickness. Localized thickening or lobulation suggests abnormality. The right middle lobe bronchus is rarely seen on the lateral radiograph.

The left pulmonary artery is visualized on a plane posterior to the trachea and carina, while the right pulmonary artery lies anterior. The right superior pulmonary vein lies anterior to the right middle lobe bronchus as it enters the left atrium. The left superior pulmonary vein lies anterior to the left upper lobe bronchus. The inferior pulmonary veins lie below and behind the superior vein and end-on visualization of these veins may simulate a mass.

The oesophagus often contains considerable quantities of air, even in normal subjects, and may therefore be visualized on the radiograph.

Anteriorly the lungs meet the anterior chest wall above the heart forming a translucent retrosternal space. This area may be opacified by a haematoma, a thymic mass, a goitre, or enlarged lymph nodes, particularly in patients with lymphoma.

CT is used routinely in the assessment of benign and malignant lesions throughout the chest wall, lung fields, or mediastinum. CT may also be used in patients with thoracic trauma, to define or confirm an abnormality detected or suspected on the standard chest radiograph. Cross-sectional imaging overcomes the difficulties of interpretation due to superimposition of structures on the chest film. CT is particularly sensitive in detecting pleural abnormalities, air, and fluid collections, which may pass unnoticed on the supine radiograph. It allows discrimination of blood from other pleural fluids due to the differences in density and is valuable in the assessment of mediastinal widening. Patients with high velocity deceleration injury and suspected trauma to the aorta or brachiocephalic arteries must always be studied by angiography to locate precisely the site of injury. However, in stable patients with mediastinal widening in whom there is a low index of suspicion of aortic injury, CT with intravenous contrast is advised. The mediastinal widening may be elucidated as being due to a fractured sternum, unfolded thoracic aorta, congenital vascular anomaly, a paramediastinal pleural collection, or a paraspinal haematoma related to a vertebral fracture. Pulmonary injury, including laceration, haematoma, and contusion is clearly defined. CT contrast studies allow differentiation of contused lung from adjacent collapsed lung or haematoma. Pulmonary abnormalities noted on CT are usually more extensive than those recognized on the plain chest radiograph. Pleural lesions, haematoma, and fractures may be demonstrated by CT but are usually apparent on clinical examination and on the plane radiograph.

CT may be helpful in the evaluation of myocardial injury. The ability to demonstrate tracheobronchial or oesophageal disruption is uncertain. Contrast examination of the oesophagus remains the investigation of choice in suspected oesophageal rupture. In patients with severe lower thoracic injury, CT scanning of the liver and spleen following intravenous contrast should be performed.

Detailed discussion of the cross-sectional anatomy of the chest is beyond the scope of this Section. The trachea and major bronchi are well visualized. The major bronchi are identified by their size and orientation. For example, the right upper lobe bronchus has a horizontal course and the bronchus intermedius passes in a vertical direction. Obliquely orientated bronchi are more difficult to evaluate. In the hilum, there is considerable variability of the vascular components. The bronchi are the most consistent structures and the veins the least consistent, in terms of their anatomical relationships. Inspection of the hilum therefore begins with the evaluation of the main lobar and segmental bronchi. The pulmonary arteries can then be distinguished from the veins by their relationship to each other and the bronchi. In the periphery it is not possible to distinguish the arteries from the veins.

The normal hilar lobulation is due to vascular structures. In particular, the superior pulmonary veins on the right are prominent components of the hilar contour and should not be mistaken for a hilar mass. The pleural fissures are rarely visualized but may be inferred by an area of avascularity. In the supine position, there is an paucity of vessels anteriorly compared to the posterior zones. This appearance is due to the effect of gravity on blood flow and may be reversed by turning the patient. The subpleural regions are relatively avascular: normal pleura is not visualized.

Anomalies of the intrathoracic great vessels rarely result in symptoms but can cause confusing mediastinal contours on the chest radiograph. Contrast CT scanning, MRI, or angiography are used to confirm the diagnosis of a vascular malformation. An aberrant right subclavian artery occurs in one in 200 people. The artery arises from the distal part of the aortic arch as its last branch, and crosses the mediastinum obliquely upward from left to right behind the oesophagus (Fig. 25) 1934. On the lateral radiograph the vessel causes a retrotracheal opacity which may be mistaken for a mass (Fig. 26) 1935. The vessel may obscure the aortic arch and cause anterior tracheal displacement. In some patients, the artery arises from an aortic diverticulum: if large, this may be misdiagnosed as an aortic aneurysm. An aberrant left subclavian artery arises from a right-sided aortic arch in approximately one in 1000 people. About 10 per cent of these individuals have associated congenital heart disease.

The most common venous anomaly is the lateral displacement of the azygos arch associated with an azygos lobe. The azygos lobe represents the medial portion of the right upper lobe trapped by the azygos fissure and vein. CT shows the azygos arch to be abnormally high, with the azygos vein joining the superior vena cava at the level of the brachiocephalic veins. Above the carina the posterior portion of the azygos vein may simulate a pulmonary nodule on CT. The lung often protrudes into the mediastinum behind the superior vena cava to outline its medial and posterior walls. The lung may also extend into the retrotracheal space, outlining the posterior wall of the trachea and displacing the oesophagus to the left. A left-sided superior vena cava is a rare anomaly, occurring in 3 per cent of normal individuals and 5 per cent of patients with congenital heart disease. Most have a right superior vena cava and the left brachiocephalic vein may be small or absent. The left superior vena cava lies lateral to the left common carotid artery and anterior to the left subclavian artery. It descends lateral to the aortic arch and main pulmonary artery and anterior to the left hilum to enter the coronary sinus posterior to the left atrium and ventricle. A left superior vena cava is readily identified on CT (Fig. 27) 1936. Azygos or hemiazygos continuation of the inferior vena cava have identical CT appearances. In both, the azygos arch, azygos vein, and superior vena cava are dilated due to the increased blood flow. The enlarged azygos system may be mistaken for a mediastinal mass or for posterior mediastinal lymphadenopathy.

In hemiazygos continuation of the inferior vena cava, the dilated hemiazygos vein may not communicate with the azygos system and will course up the left side of the mediastinum to produce a retro-aortic mass. The dilated azygos vein is often of equal diameter to the aorta. The suprahepatic portion of the vena cava develops normally, and an intrathoracic inferior vena cava is therefore usually present with azygos continuation (Fig. 28) 1937. Visualization of an inferior vena cava on the chest radiograph does not exclude the diagnosis of azygos or hemiazygos continuation of the inferior vena cava. Associated anomalies of the abdominal inferior vena cava are common and polysplenia and congenital heart disease may be present (Fig. 29) 1938.

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